1. Field of the Invention
The invention relates to a method of determining a compensation signal for the compensation of a temporally varying field strength of the main magnetic field of a main magnet of a magnetic resonance imaging device which also includes at least one gradient field coil for generating a gradient magnetic field and a magnetizable material which interacts with the magnetic fields of the device.
2. Description of Related Art
Magnetic resonance imaging devices are known per se, for example for the imaging, by way of magnetism, of a body, such as the human body, or parts of a body. In literature such imaging is also referred to as xe2x80x9cMagnetic Resonance Imaging (MRI)xe2x80x9d or xe2x80x9cNuclear Magnetic Resonance (NMR)xe2x80x9d.
A typical magnetic resonance imaging device, for example as known from U.S. Pat. No. 5,214,383, includes a receiving space for accommodating an object to be imaged. A steady or main magnetic field is generated in said receiving space by means of a magnet. In order to select a region to be imaged in the relevant object, one or more so-called gradient coils are provided so as to superpose magnetic field gradients on the main magnetic field. Generally speaking, the gradient field coils produce linear variations of the main magnetic field along the x, the y and the z axis of a cartesian co-ordinate system. In order to achieve resonance for nuclei in a selected body region to be imaged, there are provided one or more RF coils which are also capable of acting as a receiver for signals emitted by resonating nuclei.
An important condition imposed on this type of imaging apparatus is that in operation the main magnetic field should be as uniform and constant as possible during the acquisition of imaging data. Fluctuations in the main magnetic field have a direct negative effect on the imaging accuracy of the device.
For the detection of the generally comparatively slow variations of the field strength of the main magnetic field, taking place with a frequency of the order of magnitude of 10 Hz or less, the cited U.S. Pat. No. 5,214,383 describes the use of a plurality of sensors for measuring the field strength variations. The gradient magnetic field superposed on the main magnetic field by the gradient field coils is zero only at the center of the magnet, so that this location represents the ideal position for the installation of sensors for measuring the comparatively slow main magnetic field variations. In a device for magnetic resonance imaging, however, the center of the magnet is not available, because the center is situated in the receiving space for the object, for example the body of a patient to be examined.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.
Therefore, it is a first object of the invention to provide a novel and improved method for determining a compensation signal for the compensation of variations of the field strength of the main magnetic field.
In a device for magnetic resonance imaging of the kind set forth this object is achieved according to the invention in that at least one quantity which is characteristic of the temperature-dependent magnetic properties of the magnetizable material is determined, the compensation signal being provided on the basis of said quantity.
The invention is based on the recognition of the fact that the magnetic properties of the magnetizable material of the device, such as the shim iron which is used for shimming the main magnet and/or magnetizable material which is used for magnetic shielding and with which the magnetic fields of the device interact, will change under the influence of heating during use. Such changing has several adverse effects, such as drift in the main magnetic field. A compensation signal which represents the variations in time of the field strength of the main magnetic field can be obtained by determining a quantity or quantities characteristic of such temperature-dependent variations.
According to a preferred version of the method in accordance with the invention, the electric signal applied to the gradient magnetic field coil, or to each gradient magnetic field coil, is determined as the characteristic quantity. This version is based on the recognition of the fact that in practice the waveforms of the signals in the gradient field coil, or each gradient field coil, are accurately known, so that the thermal behavior of a gradient field coil is also known. This means that for a given gradient waveform, at which electric power is dissipated in the coil, the magnetic properties of the magnetizable material used therein or interacting therewith will vary in conformity with a given mathematical model because of induction effects such as eddy currents. The exact effect on the field strength of the main magnetic field can be calculated for a given quantity and configuration of the magnetizable material. This is possible notably when the main magnet is composed of superconducting or practically superconducting coils with a negligibly low power dissipation. When the main magnet itself includes a field coil which has a resistance which is not negligibly small with a view to power dissipation, the effect of the thermal behavior of the magnetizable material on the variation and the strength of the main magnetic field can be further determined by measuring a relevant further quantity which is characteristic of the variations of the magnetic properties of the magnetizable material, for example, the electric power dissipated in the main magnetic field coil.
As opposed to prior art, the determination of the compensation signal according to the invention in principle does not require a separate sensor or sensors. However, in the context of the invention the use of sensors is by no means excluded.
In another version of the invention, therefore, the temperature of the magnetizable material is measured directly as the characteristic quantity. Such a measurement is performed, for example by means of one or more appropriate sensors which need not be arranged in or at the center of the receiving space of the device. Such a temperature measurement offers the advantage that all effects contributing to the heating of the magnetizable material are cumulatively included.
The method according to the invention also has a version in which the compensation signal is determined on the basis of a predetermined functional relationship between the temperature-dependent magnetic properties of the magnetizable material and the relevant characteristic quantity or each relevant characteristic quantity.
It is notably when no direct mathematical relationship exists between variations in the main magnetic field which are due to variations of the magnetic properties of one or more of the gradient field coils, that according to another version yet of the method in accordance with the invention use can be made of a look-up table in which the relevant functional relationship is recorded. The input parameter is the measured characteristic quantity and whereas the output parameter is formed by the compensation signal or representations thereof. In the case of main magnetic field coils which are not composed of superconductors, the main magnetic field can be compensated by controlling the electrical energizing of the main magnetic field coil by means of the compensation signal determined in accordance with the invention. The main magnetic field can thus be kept constant without requiring the use of further compensation coils and the like. However, the invention can also be used for devices which are provided with supplementary coils, so-called Bo coils, for the compensation of variations of the main magnetic field.
It can be demonstrated that in a device for magnetic resonance imaging there is no difference between the optimum result of an image when the main magnetic field and a main oscillator (synthesizer) are both extremely stable or when both vary in time in the same way. Most contemporary magnetic resonance imaging devices include a synthesizer whose frequency and phase can be adjusted.
According to a preferred version of the method in accordance with the invention, variations of the field strength of the main magnetic field are compensated for the imaging result in that during use the frequency of the RF oscillator means is adapted in conformity with the compensation signal determined.
Instead of or in addition to the compensation of the frequency of the RF oscillator means and/or the main magnetic field, according to the invention it is also possible to provide a compensation signal whereby the final result, i.e. the information signal, can be compensated in respect of faults due to field strength variations. To this end, the compensation signal can suitably co-operate with processing means which deliver the ultimate information signal, so that compensation can be realized entirely on the basis of software.
In accordance with the method of the invention, the variation of the field strength of the main magnetic field can be determined and compensated, if necessary, prior to a data acquisition period. Such compensation is generally considered to be adequate for slow variations of the main magnetic field, for example as caused by the slow rise or drop in temperature of the magnetizable material of one or more of the field gradient coils or the main magnetic field coil itself. In the case of faster variations of the field strength of the main magnetic field, however, in conformity with the method of the invention the variation of the field strength of the main magnetic field can be determined and compensated, if necessary, one or more times during a data acquisition period.
In addition to variations of the main magnetic field which are due to changing magnetic properties of the field gradient coils, other sources of disturbances having an effect on the field strength of the main magnetic field are also known in practice.
Therefore, according to a further version yet of the method of the invention the compensation signal is co-determined by determination of variations of the field strength of the main magnetic field which are caused by one or more further quantities, including external magnetic fields, atmospheric pressure and vibrations exerted on the main magnet, the degree of compensation being determined on the basis of a relevant function which indicates the effect of a measured quantity on the main magnetic field.
A compensation signal thus generated is capable of eliminating undesirable variations of the field strength, affecting the quality of the image, to a high degree.
The invention also provides a device for magnetic resonance imaging, including a receiving space for accommodating an object to be imaged, a main magnet for generating a main magnetic field in the receiving space, at least one gradient field coil and at least one high-frequency (RF) coil, energizing means and control means for energizing and controlling the main magnet, the gradient field coil and the RF coil, and processing means which are actively coupled to the energizing and control means in order to determine a compensation signal for the compensation of a temporally varying field strength of the main magnetic field, which device is characterized in that the processing means are arranged to carry out the method according to the invention as described above.
The method according to the invention is suitable not only for use in conjunction with magnetic resonance imaging devices for, for example, medical applications, but essentially with any device which involves a main magnetic field and auxiliary magnetic fields which have magnetic properties which vary during operation, for example devices for spectroscopy.